1. Field of the Invention
The present invention relates to a recording apparatus for conveying a recording medium and for recording an image on the recording medium in an image forming apparatus such as a printer, a copying machine, a facsimile and the like.
2. Related Background Art
In conventional recording apparatuses for recording an image on a recording medium by using a recording means such as a recording head, there are provided a convey roller disposed at an upstream side of a recording position including a recording head and an urging member urged against the convey roller, and the recording medium is pinched between the urging member and the convey roller and conveyed by them. The position of the convey roller is fixed, and the urging member can be moved toward and away from the convey roller. Thus, a recording medium conveying surface of the convey roller is maintained at a constant level and an urging surface of the urging member which is contacted with the recording medium is rocked.
Hence, when a thickness of the recording medium is changed, a distance between the recording head and a recording surface of the recording medium is changed. That is to say, in case of a thick recording medium such as an envelope, the distance between the recording head and the recording surface of the recording medium becomes small in comparison with a thin recording medium such as a cut sheet. As a result, the recording quality is changed or the recording medium is caught by a carriage when the carriage performs main scan to thereby make the scanning operation of the carriage impossible. To avoid this, as disclosed in U.S. Pat. No. 4,755,836, there has been proposed a mechanism in which a distance between the recording head and recording surface of the recording medium is kept constant.
FIGS. 8 and 9 show such a conventional mechanism. FIG. 8 is a sectional view of a so-called serial printer in which a recording head scans along a main scan direction, taken at a plane perpendicular to the main scan direction, and FIG. 9 is a partial view of a carriage, viewed from a direction shown by the arrow A in FIG. 8.
In FIGS. 8 and 9, the reference numeral 101 denotes a carriage on which a recording head is mounted and which is reciprocally shifted in a main scan direction; 102 denotes a carriage shaft along which the carriage is shifted; 103 denotes a small roller; 104 denotes a roller lever for holding the small roller 103; 105 denotes a rail disposed in parallel with the carriage shaft 102; 106 denotes a convey roller for conveying a recording medium; 107 denotes an urging member for generating a conveying force by urging the recording medium against the convey roller; and 108 denotes a spring disposed between the roller lever 104 and the carriage 101. The small roller 103 held on the roller lever 104 is urged against the rail 105 by the action of the spring 108. As the reaction, the carriage is subjected to a force for rotating the carriage around the carriage shaft 102 in a clockwise direction (FIG. 8). A projection 101a formed on the carriage 101 is always contacted with the urging member 107 by a force for rotating the carriage toward the urging member. Thus, a distance between the recording head (not shown) mounted on the carriage 101 and the urging member 107 is maintained to a predetermined distance or more. Accordingly, even when the thickness of the recording medium is changed, a distance between the recording surface of the recording medium and the carriage (and accordingly the recording head) is always kept constant, to thereby prevent the recording medium from being caught by the carriage (i.e., preventing the scanning operation of the carriage from being impossible).
However, in the above-mentioned conventional technique, since the distance between the carriage tending to rotate around the carriage shaft and the urging member is kept constant by contacting the projection of the carriage with the urging member, when the thickness of the recording medium is changed, an amount of rotation of the carriage around the carriage shaft in an anti-clockwise direction is changed. As a result, a printing angle of the carriage (and accordingly the recording head) with respect to the recording medium is changed, so that the desired printed result cannot be obtained.
Further, since the position of the projection is shorter than the recording position of the recording head regarding the carriage shaft (rotation center), dispersion in length of the projection affected an influence upon the distance between the recording head and the recording surface of the recording medium. Thus, the distance between the recording head and the recording surface of the recording medium is changed every recording apparatus, and, thus, the recording quality is changed every recording apparatus. To avoid this, the length of the projection must be controlled accurately to keep such distance constant, to thereby make the apparatus more expensive.
Since the projection is always urged against the urging member, when the main scan of the carriage is repeated, the projection is gradually worn, so that the distance between the recording head and the recording surface of the recording medium is gradually changed, to thereby change the recording quality.